This invention relates to a process for continuous preparation of aromatic diazonium fluoride salts wherein diazotizable aromatic primary amines are continuously diazotized in a reactor containing a plurality of sequential reaction zones, such as a tubular cross-flow reactor.
Diazotization of aromatic primary amines to prepare aromatic diazonium fluorides is of considerable importance in that such diazonium compounds are intermediates which can be converted to dyes (e.g. amino azobenzene) and aromatic fluorides which are useful as intermediates in preparing various pesticides, pharmaceuticals and other products.
Processes have heretofore been proposed for continuous diazotization of aromatic primary amines. Hupfer, U.S. Pat. No. 3,117,954 discloses a process for continuous diazotization of amines wherein the diazotization is carried out in several cascaded stirring vessels or several vessels arranged in series or in combination of the two kinds of plants. The end point of the diazotization reaction is potentiometrically monitored and additional amine and nitrous acid reactants are added in response to maintain a constant measured potential.
Kindler, U.S. Pat. No. 3,423,391 discloses a process for the continuous diazotization, particularly at relatively high temperatures, of amines with diazotization agents and for the further reaction of the resultant diazo compounds or allegedly for their recovery as such. However, the patent discloses that the diazo compounds obtained are separated or further reacted so rapidly that no appreciable amounts of decomposition products can form. According to the patent, substantially all (e.g. at least 50%) of the heat of reaction from the continual mixing and reacting of the amine and the diazotization agent is absorbed by the reaction mixture, the temperature thereof rising by from 20.degree. C. to 50.degree. C. or more, and the diazo compounds formed are obtained at 40.degree. C. to 80.degree. C. or, in case of high melting point or sparingly soluble components, to 100.degree. C. or even higher. In a favored embodiment (illustrated in FIG. 1 and described in Columns 3-4 thereof) for a substantially adiabatic reaction, diazotization agent, amine or amine salt to be diazotized, and acid (if required) flow from separate reservoirs into a flow tube. According to the patent: "The materials are mixed here, for example, by means of a nozzle 11, and reacted during a short residence period and without cooling, i.e. with a rise in temperature." The diazo compounds may then be further processed in an immediately adjacent circulation system 12. Such embodiment is illustrated in Example 4 of the patent, which discloses flows from three separate reservoirs of (1) a solution of hydrogen chloride in water, (2) p-chloroaniline, and (3) a solution of sodium nitrate in water, which are mixed in the flow tube. According to such Example 4, the temperature of the three reactants upon entry into the reactor is 89.degree. C., and after a reaction period of about 0.2 second, the boiling diazo solution leaves the reactor at about 100.degree. C. In another embodiment (shown in FIG. 2 of Kindler), combination of the reacting components takes place in a very short tube by means of a nozzle which sprays the reaction mixture tangentially onto the inner wall of a cyclone type container 12 in which it flows down in a spiral path and further reacts during a short residence period. Such embodiment is illustrated for aniline in the patent's Example 6, wherein the temperature of the resulting diazo solution was 55.degree. C. in the lower part of the container.
U.S. Pat. No. 4,246,171 (Hamilton, et al) discloses a continuous diazotization process in which the rate of addition of the inorganic nitrite is controlled by a polarovoltric method. An aqueous solution of an amine in an acid and a solution of an inorganic nitrite are added continuously and regularly to a reactor. The addition of the solution of inorganic nitrite is automatically regulated to ensure that a preselected concentration of unreacted nitrous acid is maintained in the reactor throughout the entire reaction period.
Behringer, et al (U.S. Pat. No. 4,268,437) discloses a process for the continuous diazotization of primary aromatic amines by reacting an aqueous solution or suspension of the amine in a mineral acid with an aqueous sodium nitrite solution. The process comprises: supplying continuously the lower portion of cylindrical diazotization vessel placed in upright position with an aqueous mineral acid solution or suspension of a diazotizable primary aromatic amine and supplying the vessel simultaneously, via one or more inlets arranged one above the other so as to open laterally thereinto, with an aqueous sodium nitrite solution, the amine and nitrite being used in stoichiometric proportions, or the nitrite being used in a stoichiometric deficiency and the acid being used in an excess of about 1 to 3 equivalents per amine equivalent in the mineral acid solution; reacting the resulting mixture with agitation and producing a laminar flow of liquid matter at temperatures of about 5.degree. C. to 30.degree. C.; removing reaction mixture from the upper portion of the diazotization zone at a place where the reaction mixture is substantially free from nitrous acid; filtering the reaction mixture removed and delivering diazonium salt-containing solution to a sojourn zone.
As described therein, if the amine undergoes diazotization slowly, it is good practice to introduce the aqueous sodium nitrite through a plurality of inlets disposed at different levels. According to Example 2 (using 3-nitro-4-amino toluene as the amine), sodium nitrite solution was continuously supplied through three inlets in quantities increasing from above to below. This is in contrast to the disclosure in Column 2, Lines 45-49 of Behringer, et al, recommending use of quantities decreasing from above to below so as to have a minor excess proportion of nitrous acid just in the reaction mixture formed within the region of the uppermost inlet.
Trecek, U.S. Pat. No. 4,018,751 describes a continuous process for producing aminoazobenzenes in a tubular reactor. A reaction stream comprising a large excess of an aromatic primary amine, a mineral acid and water is mixed with a second reaction stream comprising an aqueous solution of an alkali metal nitrite. The combined premixed streams are then continuously passed through a tubular reactor. The exit stream from the reactor comprises aminoazobenzene compound, which is recovered therefrom. According to the patent, in the tubular reactor, a portion of the amine diazotizes to form the corresponding diazonium salt which instantaneously couples with the unreacted (excess) amine to form the diazoamino compound, which rearranges under the acidic reaction conditions to form the aminoazo compound. The residence time in the reactor is less than about 10 minutes and the reaction temperature is between 40.degree. C.-100.degree. C. Reaction time is typically about 1-10 minutes.
However, the heretofore known continuous diazotization processes have not been entirely satisfactory in that they typically are complex, inefficient, expensive, and/or prone to result in an unacceptably high level of tar and/or other byproducts especially upon reaction of the diazotized amine products thereof to various other products. Accordingly, there is a substantial need in the art for a continuous diazotization process which would overcome the disadvantages of heretofore known processes.
The present invention substantially fulfills the above need and overcomes the above-noted deficiencies of the prior art in a simple, efficient and inexpensive manner by providing a continuous process wherein an aromatic amine is continuously diazotized to a diazonium fluoride in the presence of hydrogen fluoride in a multi-stage tubular reactor with close control of temperature throughout the reaction mixture.